Well known types of angular rate sensors (gyro sensors) include: mechanical type sensors that utilize the precession of a rotating body; optical type sensors that utilize changes in time to receive laser beams travelling within a rotating enclosure; and fluid type sensors that spray a sensing gas onto a heated wire in an enclosure and detecting the sprayed gas amount variations caused by rotation of the enclosure through temperature of the wire.
Recently, there is a soaring demand for angular rate sensors used for car navigation systems, a vehicle's antiskid brake systems, balance control systems for bipedal robots and mono-wheel vehicles and other systems to detect the orientation and the attitude of vehicles. Compared with the aforementioned types of sensors, inexpensive, light and compact vibration type angular rate sensors (Patent Literature 1) are going mainstream. The vibration type angular rate sensors are configured to, when an angular velocity is exerted on an oscillator vibrating in a predetermined reference direction, detect a new vibration component (hereinafter referred to as “angular velocity vibration component”) based on a Coriolis force in a detecting direction orthogonal to the reference direction and output angular velocity information based on the detected vibration component.
In the case of a car navigation system, for example, a GPS (Global Positioning System), which monitors current location, can detect the macroscopic changes in a traveling direction of a vehicle, but cannot follow abrupt direction changes at an intersection or the like. To detect abrupt direction changes, some car navigation systems employ a method for detecting the turning movement of the vehicle in the form of angular velocity and tracing the angle of how much the vehicle has turned by using the values of the time integral of the angular velocity.
In the case of an antiskid brake system of a vehicle, as another example, a value (command value) obtained by a steering angle sensor installed in a steering wheel is compared with an output value (measured value) obtained by an angular rate sensor built in the antiskid brake system to determine whether the car is skidding. Based on the comparison results, the engine output and braking force of the respective four wheels are controlled so as to stop skidding.
In yet another example, balance control systems in mono-wheel vehicles and bipedal robots need to always control the balance of the vehicles and robots to prevent side toppling, side rolling and keep their proper attitude. The balance control system employs a method for controlling the balance with an angular rate sensor or an accelerometer as follows. When a vehicle or a robot is rotated or has the center of gravity shifted by wind, inclination, loads or other factors, the angular rate sensor or accelerometer measures the rotation or the displacement of the center of gravity to move the wheels toward the rotational direction or control an actuator positioned in the direction in which the center of gravity has shifted to generate larger force, thereby keeping the vehicle or robot in balance.
By the way, in the vibration type angular rate sensors, motion components of an oscillator in a detecting direction are not limited to Coriolis force derived from angular velocity. When the sensor is mounted in a vehicle, a robot or the like, unwanted acceleration components, such as sudden shock and vibrations caused by other factors than angular velocity, are often added to the sensor. Since such unwanted acceleration components, of course, appear as noise for angular velocity to be originally detected, it is needless to say that when the angular rate sensor is used to detect the orientation and attitude of a vehicle, the unwanted acceleration components directly cause deterioration of the detection accuracy. Patent Literature 2 and Patent Literature 3 disclose sensors having two dead weights (oscillators) combined so as to oscillate in anti-phase with each other in a reference vibration direction and outputting the difference between angular velocity vibration component waveforms of the respective sensors as a final angular velocity waveform. The oscillators oscillated in anti-phase produce anti-phase angular velocity oscillation components, but the aforementioned unwanted acceleration components appear as in-phase components. The in-phase acceleration components are offset by taking the differential waveform, and only the necessary angular velocity vibration components are left, thereby improving detection accuracy.